Treatment of boiler water



- v 1,613 701l Jan. 11,1927. R. HALL v TREATMENT UF BOILER WATER l Filed .fune 6, -1924 s sheets-sheet 2 lu 0: b "ug 200 s; o @L 'L00 s; 550 l l? 4000 Q @n 10o l o 5o loo 15o zoo` 25o soo .#750

CARBON/1758201641 @303) f/v PARTS PHP MILL/0N. @+L

115 1.40 17'5 25o zs 25o' 7' EMPERA TURE: CENT/GRAUE. WENTQR ramena-,d Jan. 11, 1927.

' of precipitates which UNITED STATES RALPH E. HALL, 0F

PATENT OFFICE.

PITTSBURGH, PENNSYLVNIA, ASSIGNUR T0 JOHN M. HOPWOD,

OF DORMONT BOROUGH, PENNSYLVANIA.

TREATMENT or BoILERWA'rE'R.

Application led June 6` Figures 1 to 3 inclusive are temperaturesolubility curves -for certain salts occurring in boiler waters;

Figure 4 contains curves representing the -calcium ion concentration in equilibrium with various solid phases at various temper- I'urcs aud umler'varying conditions;

Figure 5 contains curves by reference to which may be obtained the satisfactory carbonate radical concentrations for various concentrations of sulphate radical, and for various pressures of boiler' operation;

Figure-(3 shows` in elevation a boiler pro: vided with a sludge-separating apparatus which may be used, if desired, in conjunction with the present invention.

This application is a continuation ot my (eo-pending application Serial No. 692,804, iled February 14, 1924.

The present invention relates to the treatment of natural Waters used for steam generation, and more especially to the conditioning of boiler Water.

The elicient and economic operation of a steam boiler requires, among other things, (1) that the surfaces in contact with'the Water shall not be covered with scale; (2) that corrosion shall not occur at these Surfaces; (3) that foaming and priming resultving in Wet steam shall be inhibited; and (4) that the Waste of heated water in the blow- 'down be reduced to a minimum. These factors are dependent upon (a) the amount and nature of the soluble material in the boiler feed Water; (b) the character and amount form in the boiler as the feed water undergoes concentrations incident to evaporation; and (c) the chemical reactionsv which occur in the boiler at itsV heating surfaces. These conditions may be controlled by the maintenancein the boiler itselfof the necessary conditions of proper concentration, etc.

,i Natural Waters in practicallyall instances 'contain dissolved impurities, including oxygen and carbonl dloxide from the a1r,'and

calcium. A magnesium and sodiumv sulphates,

bicarbonates, chlorides and nicarbonates,

'the soil, as well as trates from contact with silica, salts of iron, etc. s

The primary' product of the boiler is steam. The secondary and undesirable prod- '..ctis the concentrated solution of these im- 1924. Serial` No. 718,322.

purities which resultin i the precipitation of salts 1n the boiler either asadherent scale or non-adherentsludge, necessitating 'cleaning the heating .surfaces of the boiler of scale and blowing down to remove the 'sludge and decrease the content of soluble salts.

Even if the boiler' feed water be treated to remove the scale or sludge-forming impurities, ties in the boiler saturation point, resulting in the precipitation of the impurities either as a scale adherent to the boiler surfaces (undesirable condition) or as a non-adherent sludge (desirable condition) yvhich may be removed by filtration or by blow-down.V The term soft or non-adherent scale is sometimes applied to what is here mentioned as sludge.

Calcium is usually the major metallic element of the scale and sludge-forming salts inthe concentration of such impuri-' may eventually reach the it forms a Vhard dense adherentv of calcium hydroxide or calcium silicate,

which also form a hard adherent scale on the heating'surfacesof the boiler. On the other hand. if the calcium can beprecipitated as calcium carbonate, it is preclpitated in the form of a sludge or soft scale which 'does not adhere to the heating surfaces of the boiler. A f

While the calcium carbonate is precipitated in the form of a sludge, a small amount ofit may enter into the composition of a thin film which sometimesforms'on the boiler surfaces. This thin film' consists usually of about calcite, 10% hydrons magnesium vsilicatevand 5% iron hydroxide and silica. It is soft and porous and ver/y readily permeable 'by the water. It

is properly maintaine Aas hereinafter does 'not increase 1nthiclmess when the sulphate-carbonate ratio Y scribed. It flakesv oi readily and is carried Y away'by the circulation of the water. Thisl Y.

which is detrimental to heat transfer at the The other` and usually minor impurities, such as magnesium and iron salts, silicates, etc.,'

' centration in .the boiler water is increased' boiler surfaces, and the prevention of' whichl isthe object f my treatment. K

It will be readily apparent, therefore, that the boiler water should be maintained in a' Y condition favorable to the formation of calcium carbonate and adverse tofthe formation of calcium sulphate or calcium hydroxide.

whose saturation is exceeded as their conby evaporation, 'may be precipitated either as' an adherent scale or as a non-adherent sludge. Fortunately, the ,maintenance of the ion. concentrations which-favor the for mation of calcium carbonate or other non; adherent calcium salts and inhibit the`formation of calcium sulphate and calcium hydroxide, also .favor the precipitation lof I 'theminor impurities mentioned in non-adherent form instead ofas-adherent scale.

It is a well. recognized.chemical` fact that saturation represents an equilibrium between undissolved solid and undisassociated molecules and ions of the salt in question. It is also established that the product of the concentrations ofthe ions to their appropriate common ion into a solutionwhich is at or.- near saturatioi'i results in 'the solubility exponents in such equilibrium for any given temperature is a constant known as the solubilityrproduct, which, if exceeded, results :in the precipitation ofthe solid phase. f It fol- "lows, therefore, that the introduction Vof a product being exceeded and the precipitation equivalents.

calcium hydrate so ution in contact with its solid phase may expressed as follows:

of the'solid phase of one salt.- Y

Consider asaturated solution of calcium vsulphate in contact with its solid phase at any temperature. The may be expressed as:

lequilibrium relation The l'solubility product at this temperature is l expressed by the equation: A

Kmncasol :i [054%] X [sof T'l I in which [Cax] 'and [SOg'] represent thel concentrations of calciuman'd sulphate ions, respectively, expressed in terms of chemical Similarly, thef 'I uilibrium of a saturated onoHnzcampoaHa-zori- Solid. Dissolved. v

and-lits solubility product at the temperature of the equilibrium is expressed as;

Kawai-slow] {ori- 12 Similarly, the equilibrium condition of c al cium carbonate in saturated solution in lcon! tact with its solid phase and its ,solubility product at the equilibrium temperature may sulphate ions. The precipitation makes their product again equal to the solubility product of calcium sulphatel The result of Athis ad- .dition 'of common ion on the amount of calcium sulphate which c aii be held in solution is to decrease this amount very materiallyk because the amount of-calcium `ion in solution variesv inversely-as the amount of sulphate ion. An Vincrease of sulphate ion in the boiler water,"due to evaporation, therefore, .decreases th'eamount of calcium ion which may be held in solution in the water.

It is recognized that the rvalue "of KM.;

(solubility product) for any salt at a chosenl temperature is not entirely constantv or uni affected by the ,presence of other'substances in solution, whether they introduce ai com-v mon ion or not; but the variations in-KLM thus caused are not sufficient `to alter its sigditioni'ng of boiler wateigas the concentrai' tions of the-salts in question are relatively small.

The value ofV Km;- varies largely with teinperature changes for `two reasons: (a) the concentration or total amount ofundisassociated molecules and ions in equilibrium with the lsolid phase changes with tempera# ture; (b) the per cent off disassociated molecules or ions'changes with the'temperature.

nificance for practical applicationv in the con-ll Consider the solubility curve of calcium sulphate, as shown in Figure 1 of the drawing, .i

in' which the abscissae are temperatures. of

" the solution of calcium s ulpliate'and the orf dinates fare the concentration expressed as parts permillioni Throughout this specilication, concentrations are. expressed as parts byweight o\f the soluteor' chemicalv equivalents of the solute, in parts by weight of the resulting solution. The curve a b represents `the solubility of'calcium sulphate in pure water and shows the concentrations -at the various temperatures plotted, of a satux rated solution. At` `the point a, the total ,ing to some concentration of CaS04 in the solution and also the per 4cent of disassociated molecules 1s higher than Z). Hence:

Kalpcasoi al Ul Kapcasoi itl' b 'lhc introduction ot' a coiinnon ion influences the solubiliy ot' salts of that ion. 'lhnsthc presence ot' some other sulphate in solution, as for example', sodium sulphate, will lower the solubility ot' the calcium sulphate, and its solubility curve will be lowered to a position, say to the surve (1, b, corresponddetinite concentration ot' sulphate ion; in the specific example shown in Figure l, the concentration of sulphate radical (S04) being 2,00()y parts per million. Intermediate amounts ot' sulphate radical will cause the solubility curve ot calcium sulphate to lie in positions intermediate the curves a 7) and a b.

It will be -noted that, as shown by these curves, the solubility of calcium sulphate decreases with temperature increase within the temperature range plotted.

In Figure 2, the curve c el shows therelation between the teinperatureand the concentration yof a saturated solution of calcium carbonate in pure Water, andthe curve c`d shows such relation when concentra.v tion of carbonate radical (CO3) is increased to 150 parts per million, the addition of 'sodium carbonate.' It Will be noted that the solubility of calcium carbonate increases with the temperature overthe temperature range plotted.

In Figure 3, the curve e f shows the relation between the temperature and the concentration of a saturated solution of calcium hydroxide in pure Water, and the curve e f shows such relations When concentra- .tion of hydroxyl radical (OH.) is increased to 30() parts per million, as for example,by the addition of sodium hydroxide. It` will 'be noted that the solubility of calcium.y hy# droxide decreases with the temperature over thev temperature range plotted.

From the electrical conductivity of salts ofthe different ionic types, Noyes .and his co-Workers I (Publication No. 63, Carnegiephase, and to as for example, by'

values ot calcium ion in the presence o'l",ex. cess sulphate, hydroxide and carbonate arev likewise plotted against temperature.

rl`he stable solid phase in equilibrium with the boiler water at any temperature and with varying concentrations. ot S04, C0 and OH is the one corresponding,r to the lowest satnration vaille of Cat". Thus at intersection A and the C()3 and SO ,concentration indicated, CaCQ, and CaSO4 coexist as solid phase. To the left of A, C21-CO3 is the solid the right, CaSO4 is the stable solid phase.. l/Vhen the SO4 concentration is 2,000 parts lper million and COS-is 150 parts per million, the intersection is at D. Thus a boiler, operating at 150 pounds per square inch gage pressure, or 185 C., is too close to the intersection for certain prevention of CaSOJt deposition,and either the CO3. con. centratio-n should be. increased, or the-'S04A concentration diminished. In general, inhibition ofl CaSO, deposition 4demands opera-l tion in the field to the lett ofthe intersection as determined by the CO3 and SO4 .concentrations. =Consider the adjustment of this intersecltion inthe case of calcium sulphate and callcium carbonate. At the point ofintersection,` the' solid phasesvof both calcium carbonate and calcium sulphate are in'stable equilibrium with the boiler Water, andthe calcium ion concentration in the .solubilityproduct relations of the two' materials is the same. With the calcium ion concentration term cancelled, the ratio of the solubility products of carbonate and sulphate is equal to the ratio of carbonate ion to sulphate-ion, or:

Kap. ci; S04

It is a purpose of the boiler water convdij tioning herein described to prevent the deposition of calcium sulphate as solid phase, by making the carbonate ion concentration somewhat greater than the ratio of the solubilityproductofcalcium carbonate to that ot the sulphate at the temperature of the y boiler water into the sulphate ion concentration; or what 1s the same thing, to make the intersection point, such as D, occur at la temperature higher than the maximum tem- Uo perature encountered in the boiler wat-er.

ot boiler Water In most cases, it is easiest and most ecorium carbonate, whenv used, not lonly decreases the sulphate concentration by prenomical to maintain this co'ndition by introcipitating'ibar-ium sulphate, but also may furnish all or a partof the carbonate radical concentration required. v

For simplicity, the isohydric principle has ynot been used in calculating the curves of Ficure 4, and hence, points of intersection Af-B, C, and D are 'in slightly dierent position`fronr what they would be if the isohydric' principle had been/taken into account. 'This variation is not inore than the usual `limits of variability of the carbonate co-ncentration which can be maintainedin boiler operation. The curves of Figure 5 have been developed with full consideration of the isohydric principle. It Will be understood, of course, that Figure 4 is presented for purposes of explaining ythe invention, whereas Figure 5 illus-trates the type of curve which is actually used incomputing the amount of carbonate radical to tained in the boiler Water.

Under boiler conditions, the rapid vdevelopment of steam results`in anexceedingly small partial pressure of carbon dioxide in the .gaseous phase in contact with the'4 boiler lWater, fand gaseous carbon dioxide passes off VWith the steam, and hydroxyl ion l 4is introduced 'into the boiler `vvater by the ,hydrolysis of the sodium carbonate and re- I In practice, to maintain this condition, either the concentration of the hydroxyl ion may be, on the one hand, diminished by car;

bonic acid introduced by the addition of a bicarbonate or byothe 4addition of carbon 'di-l oxide, or,jon theother hand, by diminishing the sulphate ion concentration, as by the use of a barium compound whereby the concentration of carbonate ion need not become so ,great and thereby the' development of hydroxyl ion concentration, be-retarded.

At temperatures above 200 C. (210 pounds."

' honate, should be added to the boiler water.

4The sodium carbonate ields carbonate ion which precipitates calcium 'as calcium car-.- bona'te. It also yields hydroxyl ion. wl1ichprecipitates ma esium as magnesium hydroxlde. i The ydroxyl ion also tends to minimize the hydrogen ion concentration at if a 1r indiscriminate excess of sodium carthe metal surfaces and thus prevent corrosion. On the other hand,'a; too great hydroxyl ion concentration, which will occur be mainshall not be Kut-Cba at temperature of less-than KLM-(03), boilerWatertim'es sulphate,

limited mains in the water.` This condition is not disadvantageous as the tendency of'hydroxyl cuz-red in the examplel just presented of cali cium sulphate. Thus in Figure 4:, B represents a ypoint at which calcium carbonate and calcium hydroxide coexist as. solid phase, when the intersection D occursrto the right of B, that is, vvhen the sulphate concentration is low enough so that calcium hydroxide is precipitated in preference to calcium sulphate. To the leftv of B, calcium carbonate, and to the right, calcium hydroxide, is the stable solid phase. The latter condition is undesirableas calcium hy.- droxide forms adherent scale. It is desirable, therefore, to so regulate the de velopment of hydroxyl ion concentration 1n the boiler water that intersection point B shall occur ata temperature. higher than that encountered in the boiler water.

The necessary criterion to maintain in the boiler Wabonatev'i's added, vvill'cause the precipitatin of calciumv hydroxideas an encrusting scale. Moreover, hydroxyl ion -in its higher concentrations is an excellent peptizing and saponifying agent, and will accelerate foam- 'ingnarticula-rly if there'is colloidally'dispos material or saponiliable organic matter in the water.- f l y' It is a matter of common observation that .the-most rapid deposition of hard adherent scalev 'in boilers occurs on the more highly heated surfaces of the boiler. The reason for this is apparent from a consideration of the temperature solubility curves of calcium calcium hydroxideand calcium car reaches the maximum ltemperature attained 1n the boiler, and this'temperature maybe considerably above .the average vtemperature ofthe water or the; temperature of the water' a t the holler surfaces which are not 'in proximity to the ame from the combustion chamber. As the water is increasediii temperatureby'contact 'with'a higher heated surface of the boiler, the calcium sulphate and calcium hydroxide have their solubilities decreased, while the solubility of the calciurn carbonate is increased. Therefore, while at the temperature of the main body ouate. The water in immediate contact with' the most highly heated boiler surfaces ofwater inthe boiler, the calcium sulphate and calcium hydroxide arel not precipitate- 1 they may be precipitated at the heated surface. vSince the solubility 'of the calcium carbonate is actually increased with tem' Y perature,

the carbonate ion exerts less inhibitation of calcium iting .effect on the-preci roxide at the heatsulphate and calciuniihy ed surface than in the rest of the boiler. On

the other hand,- s'ince thelsolubilities of c'alcium-sulphate and calcium hydroxide lincreasel wlth falling temperature and the solubility of calcium carbonate decreases with falling temperature, there is no tendency to precipitate the calcium' sulphate or hydroxide in the cooler portions of the boiler, but

y instead the calcium carbonate is there precipitated. Moreover, since v the coolest regions of the boiler" are not at the ,boiler surfaces, vthe calcium carbonatelis precipitated fas a sludge in the boiler Watrer 'instead of as precipitationcause an adherent scale; While -a salt which has its solubility'increased with the temperature will'not be precipitated at a heated surface, and therefore, can not form P p ily hydrolyzes encrusting scale. p

A solubility temperature curve showing a decrease in solubility with the temperature increase, therefore, may be spoken of as an adverse or unfavorable curve, While a solubility temperat-ure curve showing an increase in solubility with the temperature increase may be spoken of as a favorable curve. Similarly, anion,.such for example, as the negative sulphate, hydroxyl, carbonate or phosphate ion, may be spoken of as an adverse or favorable ion, depending upon Whether the solubilities of their calcium and Cil Cil

magnesium salts decrease or increase, respectively, with risingtemperature'.

In the preceding discussion, reference has been made primaril to calcium salts.l Itis unnecessary to dea specifically with other salt-s which may occur in the boiler water, because those conditions which must Ibe maintained to inhibit the precipitationv of calcium sulphate or calcium hydroxide meet the criteria which are necessary for lthe pre-r4 vention or minimization of the precipitation of other unfavorable salts, as for in. stance, magnesium andcalcium The breakmg down of the carbonate to form hydroxyl is always sufficient to msure the i [CO5- t] taking 4into accountof the isohydric princiand calcium'hy roxide form'an adherA precipitate against asilicate.L

at temperature of boiler Water precipitationof magnesium ion in thc form ofi magnesium hydroxide in large measure, largely in'suspension, even though its solubility curve is slightly unfavorable because of its' very small solubility in the presence of excess hydroxyl; and the concentrations of 'carbonate and hydroxyl which are attained inhibit -or minimize the precipitation of silicates.

Throughout this discussion, the term salt is given the significance of a material which consists of a positive metallic component and a negative component, and is ernployed as a term of general definition to include calcium and magnesium hydroxides as Well fas the sulphates, carbonates, etc.

lSince the carbon dioxide is continuously drained olf with the steam, the carbonate fhibt the precipitation of the calcium and magnesium as encrusting salts, other favorable 'negative ions may be used, such for example` as phosphate Jion introduced in the form of trisodium or other phosphate. Calcium phosphate has a low solubility. The hosphate, such as sodium phosphate, readto introduce hydroxyl ion into the Water, which will prevent the precipitation of magnesium as an encrusting silicate. The magnesium is precipitated as a non-adherent phosphate or hydroxide.

The proper conditioning of a boiler wa ter necessitates that the intersectionpf the calcium carbonate (or phosphate) andsulphate curves be adjusted according to the pressure at which the boiler is operated. In Figure 5 are plotted the curves showing the relationship between carbonate and sulphate which should be maintained at various pressures, to fix the intersection at such pressures. The ordinates represent gage pressure in pounds per square inch 'and cover the range customarily met with in b'oiler operation.v The statement of gage pressure is, of cou-rse, equivalent to the statement of temperature of the boiler water.

The abscissae represent the amount of carbonate radical in partsper million of solution which should be present inthe boiler water. The various curvesrepresent deli-l .,nite amounts of sulphate radical in parts per million of the solution. The curves are derived by the use' of the formula:

ple of ionization and the4 variability of-the solubility products involved with temperature change. For concentrationsA of sulphate radical other than those iven in the curves, it is eas to interpolate etween the given curves. he curves of Figure 5 are plotted in'accordance with the best chemical data at present available and are sufficiently accurate for practical boiler o ration. Actual operating experience wit steam boilers supplied with carbonate in accordance with the curves of Figure 5 have shown the amount of carbonate there indicated to be suilicient `for satisfactory scale prevention. The curves shown in Figure 5, or fi similar set ofcurves, are Afurnished to thc boiler operator. He determines the sul hate and carbonate concentration -in the filer walter., This determination may be made in any of the vusual Ways, although I prefer to use the method and type of apparatus disclosed in the article by Hall, Fisher and Smith,'published in the Proceedings of the .Association Vot' Iron & Steel Electrical Engineers for June, 1921.1, vol. l, pp. 312-327.

Having determined the sulphate radical concentration, the boiler operator refers to vthe set of curves and reads from it ,the

amount of `Vcarbonate radical necessary to prevent the precipitation of calcium sulphate scaleat the sulphate radicalconc'entration andthe boiler-pressure in question. He then introduces into the boiler water, either directly. into A'the boiler or into the feed water, a sufficient amount of carbonate radical, usually inthe form of sodium carbonate or sodium bicarbonate, lto build up the carbonate radical in the boiler to somewhat more than the theoretical amount indicated by the curves shown in Figure 5'. For example, if a boiler is operatingat V150 .pounds per square inch gage pressure (185 C.) and the sulphate determination indicates 1000 parts per million, then the essential condition for the prevention of adherent fj scale formation is that the boiler water shall contain a little more than 90 arts per million of carbonatef radical-a li le more, be- .cause if lonly parts per millionis present,

it is directly on theintersection of tlie curves, and either calcium sulphate or carbonate-inay be the solid phase; While With more than 90 parts per millionV the, solid phase can .be only calcium` carbonate. If the gage pressure is 210 pounds per squaref inch, and sulphate is 1,000 parts permil-H lion, then the Vcarbonate concentration must be slightly greater thanv 155- parts per million, but if sulphate is 750 parts per million, the carbonate need be but slightly greater than 120 parts per million.' The amount' of carbonate radical introduced should be somewhat in excess ofthe theoretical amountV I findicated in Figure 5, not only asa factor of safety, but also because of the fact thatl the waterl in: contact with the heating surpriate indicator,

faces of the boiler may be at a somewhat higher temperature than the temperature. corres nding to the gage pressure. In large` ilerstlie depth of water may be as great'as 30' or 40'feet, and the head of watier at the bottom of the boiler is 'such that the temperature at 'the bottom of the boiler may be somewhat higher than that indicated bythe gage pressure.

With aset ofcurves such as this, and simple apparatus inthe boiler room for the de termination of sulphatel and carbonate concentration, the boiler water can be conditioned so that .adherent scale .formation is inhibited. No analysis of the feed water is necessary or desirable, other than to test it for acidity now and then with an ap rosuch as brom phenol lue or methyl orange. 'I -f The preferred procedure is for the boiler operator to test the water from time to time in the boiler to determine the sulphate radivmy bonate concentration.- f'

.The testing of the boiler water gives an -Y immediate and reliable indication of the ultimate concentration of sulphate radicalV in the boiler caused by the introduction of fresh feed water, the concentration of the feed Waterby steam evaporation and the" removall of blow-down water, and is, in generalJ the mostl satisfactory Way to determine-the'ult-imate sulphate radical concentration which must be contended with. The 110.

ultimate sulphate radical -concentration which must be" taken care of may, lowev be determined by more indirect methods, for example, the feedo water may be tested` to determine lthe amount of sulphate radical contained therein, and then the concentrav tion of the feedwater which takes place in the boiler ma" be determined by a deter-- minationof t e\clilorine concentration in feed and boiler water, forby knowledge yof! the amount of'water evaporated, the'moisture in the steam, and blow down. If -we -know in any particular casethe numbeifiof concentrations which the feed Water under' roes in making the bo'ler Water, 'and if We 25 know the amount ofV ecomposition of the carbonate radical into hydroxyl radical durfing this process of concentration, then it becomes possible to introduce into the feed water the amount of carbonate radicalwhicjh 13 to month or even ferred to check the sulphate and carbonate radical concentrations in the boiler water from time to time, and to regulate the amount ot' carbonate introduced accordingly. For example,- in Jr'eed water taken from rivers, the amount of sulphate concentration Will vary considerably from month from day to day, and such variations are in'nnediately vchecked bythe sulphate analysis of the boiler water.

The development. of' (')H ion in the boiler water at the expense of GO, ion has been discussed. In View lot the C03 concentration which must be maintained, the amount of OH ion developed is ample at all times to insure the precipitation of Mg largely as `Mg(OH)2, since its solubility product is so small, probably approximately 0.015 at 185 C., and thus largely prevent any silicate scale formation. In fact, care must be taken that the OH concentrationl does not rise high enough so that Ca(OH)2 is precipitated in place of calcium carbonate. vIt' the sulphateradical concentration in the boiler water is 3,000 parts per million, reference to Figure 5 discloses that at 150 .pounds per square inch gage pressure, slightly more than 270 parts per million of carbonate radical will be necessary` in 'the boiler water to insure the precipitation of calcium as non-adherent calcium carbonate. In case the sulphate concentration is only, 2,000 parts per million, then the concentration of carbonate radical need be but 180 parts per million. But while Ks. ,',.Ca(OI-I) a concentration of 1050 parts per million ot OH in the Erst example, in which the concentration of C()3 was slightly greater than 2T() parts per million, and ot'A 85() parts per million in the second exainple, in which slightly greater than 1S() parts per million, would be sufficient to result in the deposition of Ca (OHL, if the concentration'of the CO., were perl'nitted to tall below its limiting value. i j

The hydroxyl concentration in the. water in the boilecniay bechecked, from time to time by ordinary titration methods. Usually with boilers operating at or lbelow 150 pounds per --sqnare inch gage pressure., the hydroxyl'.'feoncentration" will not rise sufliciently 'to cause any danger of precipitating the calciuin hydroxide 'as adherent ale.

ietw'een thepressure ot 150 and 200 pounds per square inch, the hydroxyl concentration should be observed andainay be regulated it 4rises to ,too high a concentra- `tion by using sodium*bicarbonate as a the carbonate, concentration was` 23, approximately,

source ot carbonate ion.'V

prefsure ot 200 pounds per square inch, it will be advisable in most cases to use,vnot arbonate radical, but phosphate radical, because the development o't' hydroxyl radical does not proceed ast-ar as with carbonate radical, since the phosphate radical is more stable at higher temperatures. At the higher temperatures, owing to the exceedinglyslight partial pre, sure ot carbon dioxide in 'the vapor phase in the boiler and to the augmented rate ot' decomposition with temperature increase, the'decomposition ot sodilm carbonate is so rapid that it would be impossible to maintain'l'he requisite carbonate concentration to cause the precipitatioIrof the calcium as ,calcium carbonate, irrespective ot the excessive. hydroxyl concentration caused by ,such decomposition. why the phosphate radical is preferable at the higher pressuresI is due to the fact that as. the gage pressureo't' the boiler increases, the amount ot' favorable radical, Such as carbonate or phosphate, which is necessary to prevent precipitation ot' calcium lsulphate, lso increases because ot'v the continued'decrease in the` solubility with increasing pressure,.an ,l the consequent' necessity ot' maintaining a greater excess ot favorable ion, in order to'. maintain the intersection ot the sulphate and the favorable ion curve in a position to the right ot the pressure at which the boiler is operated, as indicated on Figure 4. 'lhc curve of Figure 5 can be used when sodium bicarlmnate .is used, as well as' when sodium carbonate is 1tsed, since both of these introduce 'the carbonate radical for which Figure 5 is plotted. as by theI introduction o't' trisodium phosphate, a similar setof ,curves developed according toy .the for the curves ot' Figure 5 may be made. up and used in like manner.- ln my cepending application,` Serial No. 351, tiled .lannary 3, 1925,-1 have presented claims directed -more particularly' to the use olphosphate or similar stable radical.

The eonditioning'ot the boiler water may. befcarried out in various 'ways and may be eonilbined with pretreatment and filtration' or both. The simplest procedure and one which may be. carriedout with relatively pure. waters is the intrtuluction of' thel soda ash or other treating chemical directly into they boiler, .as required. andthe removal of the ,sludge and the limitation ot' the concentration 'ot soluble-salts by-blow-down. lt desired. the. ieed water before introduction into the boiler may "be given a` primar \v treatment to remove thecalcium and other salts to the limit of their*solubilities in the teed water. and the soda ash or other chemical for secondary treatnieut (that is, the

treatment for prevention ot calcium sulphate it second reason lVhen phosphate ion is intrmluced,l

principlesl already indicated Above the gage ot calcium sulphate lll) scale formation), may be introduced into the boiler, or the feed water may haveadded to it, before introduction into the boiler, a sufficient amount of soda ash for both full primary and secondary treatment. In the broader aspect of the invention,it is immai terial where the soda ash or other chemical is introduced into thel water, so long as the conditions 'for the prevention of encrusting scale precipitation are continuously maintained.

The conditioning' treatment may be advantageously combined with sludge separation from a portion ofthe boiler watertaken' .from the boiler and returned to it after removal of the sludge therefrom. The combi-l nation of these. treatments gives any desired control of the concentration of suspended solids in the boiler water.

In Figure 6 of the drawings is illustrated a sludge-separating apparatus attached to a boiler for withdrawing a part of theboiler water contaminated with sludgeseparating the sludge therefrom `and returning the heat ed water, freed fromr its sludge content, to the boiler.

In the drawings, reference numeral 1 indicates a boiler shown diagrammatically. A

' sludgey separatoi' 2 is connected with the boiler by pipes 3 and 4', through which the` water is continuously7 withdrawn and returnedto the boil-er by means of a pump 5.

The water is withdrawn through the pipe 3 tration of the suspended solids is greatest,l and the water freed from the sludge is prefwater passing through it.

from a place which is preferably near the water lineof the boiler where the concenhot alkaline boiler water, such, for example, las iron oxide, or a silicate in which the acidic character ofthe' silica is already fully neutralized by a metal component, such, for

example, as` the olivine minerals, as described in my copending application, Serial No. 710,740, filed May 3, 1924, for filtration of' alkaline waters. A centrifugal separator may be used, if desiredto separatethe sludge.'

` The increment of suspended solids in the boiler water -is'a function of such potential solids in the. feed water, and the amount of feed water which enters, together with uthe amount 'of blow-down and moisture loss 1n the'steam. Byfremoving varying amounts,

of boiler water continuously and passing v them through the sludge-removing device,

any condition desired can be maintained in the boiler water as regards the amount of suspended solids. Thus, in a boiler with 1% i point. for holding a solution of the treating chemi- `the sludge-separating `be introduced .at

'blow-down, `in which the potential suspended solids in the feed water are 170 parts per million and the inflow of feed water is 150,000 pounds per hour, if nothing but blow-.down is used, the suspended solids will eventual] reachV approximately 17,000 parts per miliion. It, however, a quantity of water equal to 5% ,of the feed wateris passed through the sludge-removing device yand all sludge removed, the concentration of upon an assumption' of complete suspension' of the insoluble solids in'the boiler water and complete removal thereof by the sludgeseparating device. These assumptions' will be rather closely approximated under actual conditions.

The separation of the sludge from waterv drawn and returned to the boiler may be effected without much loss oflieat from the boiler wat-er and will greatly reduce the amount ofv water required for blow-down.

The treating chemical, lsuch `as the sodium barbonate, may be introduced atl any desired In the drawings, a tank 6 is shown cal. The solution may be continuously or intermittently pumped from this tank by a pump7 and introduced through a pipe 8 into the water, just prior to its I yfdevice 2, or the solution may be pumped through a pipe 9 and introduced into the boiler Awith the feed waterfrom the feed` water pipe '10, or may any desired place in the boiler through an entirely separate pipe, such as shown for example, .at 11.

The present invention may be appliedto theftreatinent of water in boilers other than to the pressure boilers used forpower or heating purposes. For example, it may be `employed in the devices commonly .called evaporators, but which ,ar'e,`,-in fact, really boilers in that .theyI cause an evaporation of the'water which is afterwards condensed as .a distilled water for,7 use lin powcr-gencrating boilers supplying condensing engines provided with efcient condenser systems.

atvhigher temperatures,so that it may be;

advisable to introduce hydroxyl directly as by the addition of sodium hydroxide.

entry into VThese evaporators may he operated. under*v 'understood that theinvention is not limited to such deta'ils or to the .preferred manner'- of procedure above outlined, but that the invention may be embodied in other methods of. procedure Within the scope of the following claims.

I claim:

l. The process of preventing the formation. of adherent scale by steam. boiler Water containing a scale forming metal of the second group of the periodic system and a negative ion favorable to the formation of adherent scale, which comprises maintaining in the Water in the boiler a concentration vof a negative ion favorable to the .formation of non-adherent sludge not less than the concentration ofthe negative ion-favorable to the formation of scale times the ratiol of the solubility product of the second group metal compound of the ion favorable to the formation of a sludge to the solubility product of the second group metal compound of the ion favorable to the formation of scale, all to their appropriate exponents, at the temperature of the Water in the boiler.

2. The process of preventing the formation of adherent scale by steam boiler Water containing calcium and sulphate, which comprises maintaining in the Water in the boiler a concentration of a negative ion favorable to the formation of non-adherent sludge not less than the concentration of the sulphate ion times the ratio of the solubility product of the calcium compound of the ion favorable to the formation of sludge to the solubility. product of calcium. sulphate, all to their' appropriate exponents, at the tempera- A ture ofthe Water in the boiler.

v than the concentration of sulphate `ion times` 3. `The process of preventingthe formation of adherent scale by steam boiler Water containing calcium and sulphate, Which comprises maintaining in the Waterin the boiler a concentration of carbonate ion not less carbonate, to the solubility product of calcium sulphate, at the temperature of the Water in the boiler'.

4. The process ofthe preventing the for# mation of adherent scale by steam' boiler Water containing calcium and sulphate,

which comprises 'maintaining in the Water -,in the boiler such. concentrations of carbonate'ion and hydroxyl ion that the concentration -of carbonate ion is not less than the tio of the solubility product of calcium carbonate to the solubility product of calcium sulphate and not less than the square of the concentration of hydroxyl ion times the ratio of the solubility product of calcium carbonate to the solubility product'of calcium hydroxide, at the temperature of the Water in the boiler.'

. 5. The process of preventing the formation of adherent scale by steam boiler 'Water containing calcium and sulphate, which comprises maintaining in the water in the .boiler such` concentrations of hydroxyl ion and a negative ion favorable to the formatlon of non-adherent sludge that the convconcentration of sulphate ion times the racentration of the negative ion favorable to the formation of non-adherent sludgeis not less than the concentration -of sulphate ion times the ratio of the solubility product of the calcium salt of the ion favorable to the formation of sludge to the solubility product ofcalcium sulphate, all tol their appropriate exponents, and not less than t-he concentration of hydroxyl ion times the ratio ofthe solubility product of the calcium salt of the negative ion favorable to the formation of av sludge to' the solubility product of calcium hydroxide, all to their appropriate exponents, at the temperature of the Water in the boiler. 6. The process of preventing tl'ie formation of adherent scale by steam boiler Water containing calcium and sulphate., vWhich comprises introducing sodium carbonate intov theI Water in regulated amounts so as to maintain in the Water in the boiler the concentration of carbonate ion not less than the concentration of sulphate ion times the ratio'of the solubility product of calcium carbonate to the solubility product of calcium sulphate. and not less than the square of the concentration of hydroxyl ion times lthe ratio of the solubility product of calcium carbonate to the' solubilit-y product of-calcium hydroxide, at the temperature of the Water in the boiler. I

7. The process of preventing the formation of adherent scale by steam boiler Water containing calcium and sulphate, which comprises continuously maintaining in the Water in the boiler such relative concentrations of calcium ion, sulphate ion and a negative ion favorable to the formation of a non-adherent sludge-forming calcium salt that the calcium is precipitated as a nonadherent sludge instead of calcium sulphate scale.

8. The process vof preventing the-'formation of adherent scale by. steam boiler Water containing calcium and sulphate, which comprises continuously maintaining in the Water in the boiler such relative concentrations of calcium ion7 sulphate ion, hydroxyl ,ion and a negative ion favorable to the formation of a non-adherent sludge-forming calcium salt that the calcium 1s precipitated as a non-adherent sludge instead of a calcium sulphate or a calcium hydroxide scale.

9. The process of preventing the formation of adherent scale by steam boiler Water containing.` calcium andsulphate, which comprises introducing into the Water material yielding in solution a negative ion favorable to the formation of non-adherent sludge-forming calcium salt in such amount as to maintain in the Water in the boiler such relative concentrations of calcium ion,y sulphate ion and the negative ion favorable to the sludge formation that the solubility product of the calcium salt of the negative ion favorable to sludge formation will be exceeded before the solubility product of calcium sulphate is reached at the temperature of the Water in the boiler.

10. The process of preventing the formation of adherent scale by steam boiler Water containing calcium and sulphate, which com- 36 prises introducing into the Water a salt the water in the boiler.

11. The process of preventing the formation of adherent scale by steam boiler Water containing calcium and sulphate, which comprises introducing into the water'an alkali carbonate and carbonio acid in such relative amounts as to cause the precipitation in the water in the boiler of calcium carbonate 'as a non-adherent sludge instead of v adherent scale-forming calcium sulphate or calcium hydroxide.

i l2. The'process of'preventing the formation of adherent scale by steam boiler water .containing calcium and sulphate, which comprises determining the sulphate concentration of the water and on the basisV of such determination introducing intoV the Water material yielding in solution a negative ion favorable to the formation of a non-adherent sludge-forming calcium `salt in such amounts asY to cause precipitation of the calcium in the water 1n the'boiler as a noni-- adherent sludge instead of calcium sulphate scale.

13. The process of preventing the formation of adherent scale by steam boiler Water containing calcium and sulphate, which comprises determining the sulphate concentration of the water d on the basis of such determination introducing into the water material yielding in solution a negative ion favorable to the formation of a non-adherent sludge-forming calcium -salt in such amounts as to continuously maintain in 'the Water in the boiler such 'relative concentrations of calcium ion, sulphate ion and the negativeion favorable to sludge formation" that the solubility product of the calcium salt of the negative ion favorable to sludge formation will be exceeded before the solubility product of calcium sulphate is reached at the temperature of the water in the boiler.

14. The process of preventing the formation of adherent scale by steam boiler water containing calcium and sulphate, which comprises introducing into the Water material yielding in solution a negative ion favorable to the formation of a non-adherent sludge-forming calcium salt,from time to time determining the concentrations in the boiler Water of sulphate and the negative radical yielding such negative ion, and on the basis of such determinationsadding as required 4more material yielding the negative ion favorable to sludge formation in amounts suiicient to continuously maintain such relative concentrations of said negative ion, calcium ion and sulphate ion that the solubility product of the calcium salt of the ion favorable to sludge formation will be exceeded before the' solubility product o1 calcium sulphate is reached at the temperature of the Water iu the bgiler.

15. The process of preventing the formation of adherent scale by steam boiler water containing calcium and sulphate7 Which comprises introducing4 into the water material yielding in solution hydroxyl ion and a negative ion favorable to the formation of a non-adherent sludge-forming calcium salt, from time to time determining the concen-Y trations in the boiler Water of sulphate, hydroxyl and the radical yielding the ion favorable to sludge formation, and onthe balUU sis of such determinations introducing as r equired more material yielding hydroxylion and the negative ion favorable to sludge formation in such amounts that the solubility product of the calcium salt of the vion favorable to sludge formation will be exceeded before the Solubility products of calcium sulphate and calcium hydroxide are reached at the temperature of the water in the boiler.

l 16. The process of preventing the formation of adherent scale by steam boiler water containing calciumand sulphate, which comtion of adherent herent scale,

' tion of adherent scale by steam boiler prises introducing into the Water` material yielding in solution an ion favorable to the formation of a non-adherent sludge-forming calcium salt in precipitation of such calcium salt as sludge in the boiler, continually removing a por-v tion of the Water from the'boiler, vseparating the sludge therefrom, and returning the Water to the boiler. g

17. The process o-f preventing the formation of adherent scale byl steam boiler Water containing calcium and sulphate, which comprises introducing into the Water sodium carbonate in sufficient amounts to cause the precipitation of calcium carbonate as sludge in the boiler, continually 'removing a portion of the Water from the boiler, separating the sludge therefrom, and returning the Water to the boiler.

18. rlhe process of preventing the formascale by steam boiler Water containing a scale-forming metal of the second group of the periodic system and a negative ion favorable to the formation of ad- Which comprises maintaining in thev Water in the centrations of the second group metal, the negative ion favorable to the formation of adherent scale and a negative ion favorable to the formation of a non-adherent sludgeforming salt that the second group` metal is precipitated as a non-adherent sludge in-v stead of adherent scale.

19. The process of preventing the forma- Water containing calcium and sulphate, which comprises determining the sulphate concentration of the Water and on the basis of such determination introducing into the Water carbonate in such amount as to cause the precipitation in the Water in the'boiler of the calcium as a non-adherent calcium carbonate Ysludge instead of calcium sulphate scale.

20. The process of preventing the formation of adherent scale by steam boilerA water containing calcium and sulphate, which com-- prises introducing carbonate `into the water, from timeto time determinmg the concentration of carbonate and sulphate in the v vaadding as r requiredmorev 'amount sufficient to maintain 1n the Water` suiiicientamounts to cause the-5in the boiler such relative concentrations boiler such relative con-- determination ter, and on the basis o f such carbonate 1n of calcium carbonate andvsulphate that the solubility product of calcium carbonate will be exceeded before the ture of the Water in the boiler.

2l. The process of preventing the formation of adherent scale by steam boiler Water containing calcium, magnesium, sulphate and silicate,` which comprises maintaining in solubility product of calcium sulphate is reache'dat -the temperathe Water in the boiler a concentration of hydroxyl ion and a negative ion favorable to the formation of a non-adherent calcium salt, such that the magnesium and calcium are precipitated as sludges instead of an adherent scale. v f

22. The process of preventing the formation of adherent scale in steam boilers using feed Water containing calciumand sulphate, which comprises decreasing the sulphate by means of a barium compound and maintaining in the Water in the boiler a concentration of a negative ion favorable to the formation of non-adherent sludge not less than the concentration of the sulphate ion times the ratio 0f the solubility product of the calcium compound of the ion favorable to the formationy of sludge to the solubility product of cal-4 cium sulphate, all to their appropriate exponents, at the temperature ofthe Water in 'the boiler.

23. The process of preventing the formation of adherent scale in steam boilers using feed Water containing calcium and sulphate which comprises decreasing the sulphate by means of a barium compound and maintaining in the water in the boiler a concentration of carbonate ion not less tlan the concentration of sulphate ion times the ratio of the solubility product of calcium carbonate to the solubility product of calcium sulphate, at the temperature ofthe Water in the boiler.

' In testimony whereof I have hereunto set my hand.

RALPHv E. HALL. 

